1. Field of the Invention
The present invention generally relates to a nonvolatile semiconductor memory device and, more particularly, to a nonvolatile semiconductor memory device using a reference cell upon reading data.
2. Description of the Related Art
Generally, in a nonvolatile semiconductor memory device, such as a flash memory, a current of a reference cell set at a predetermined threshold value is used as a reference current, and upon reading data, a drain current of a read memory cell is compared with the reference current. According to whether or not the drain current of the read memory cell is larger than the reference current, the data is judged to be xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d.
As data in the flash memory is rewritten number of times, a charge loss occurs in which a write charge is lost. Due to this charge loss, a threshold value of the memory cell of a core circuit tends to become smaller. By contrast, the threshold value of the reference cell remains fixed because data in the reference cell is normally not rewritten. Therefore, as the rewrite number of times increases, the fixed threshold value of the reference cell results in a read margin being unable to be secured sufficiently.
For the purpose of sufficiently securing the read margin, two types of reference cells are prepared as read reference cells for data xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d, respectively, and an average current therebetween is used as the reference current, in which the same rewriting operation is performed not only to the memory cell but also to these reference cells. That is, in this method, a programmed reference cell Ref0 for data xe2x80x9c0xe2x80x9d and an erased reference cell Ref1 for data xe2x80x9c1xe2x80x9d are prepared, and an average current between two reference currents thereof is used as a read reference current.
In this method, the reference cell Ref0 is programmed in the same manner as the memory cell by using a program-verify reference cell PrRef set at a predetermined threshold value. In this program verification, the program-verify reference cell PrRef is compared with a write cell to which data is being written; then, when the write cell exceeds the threshold value of the program-verify reference cell PrRef, the writing of the data is ended. Thus, the reference cell Ref0 programmed as above is provided with only a lower limit of a threshold value, and the threshold value per se is indeterminate. Similarly, the reference cell Ref1 is provided with only an upper limit of a threshold value, and the threshold value per se is indeterminate.
As described above, the threshold values of the reference cells have probability distributions to a certain extent depending on writing/erasing properties thereof.
A distribution of a threshold value of a virtual read reference cell, which is an average between the two reference cells, exerts a large influence upon the read margin. When the threshold value of the virtual read reference cell is high, a difference from a threshold value for data xe2x80x9c0xe2x80x9d of the memory cell becomes small so that a margin to data xe2x80x9c0xe2x80x9d becomes small. On the other hand, when the threshold value of the virtual read reference cell is low, a difference from a threshold value for data xe2x80x9c1xe2x80x9d of the memory cell becomes small so that a margin to data xe2x80x9c1xe2x80x9d becomes small.
As described above, when an average current between two reference cells is used as a reference current, the average current varies, because threshold values of the reference cells vary depending on writing/erasing properties of the reference cells; accordingly, the read margin becomes unstable.
It is a general object of the present invention to provide an improved and useful nonvolatile semiconductor memory device in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide a nonvolatile semiconductor memory device using an average current between two reference cells as a reference current with securing a stable read margin.
In order to achieve the above-mentioned objects, there is provided according to one aspect of the present invention a nonvolatile semiconductor memory device including a memory cell, first and second dynamic reference cells subjected to a same rewriting as performed to the memory cell, and a data judgment control circuit obtaining an average reference current from the first and second dynamic reference cells, and comparing a current of data read from the memory cell with the average reference current so as to judge the data, wherein the data judgment control circuit programs the second dynamic reference cell according to a threshold value of the first dynamic reference cell.
According to the present invention, the data judgment control circuit programs the second dynamic reference cell according to the threshold value of the first dynamic reference cell. Therefore, the second dynamic reference cell can be programmed so that a threshold value of the second dynamic reference cell becomes large when the threshold value of the first dynamic reference cell is small. Thus, a stable read margin can be provided for a nonvolatile semiconductor memory device using an average current between two reference cells as a reference current with securing.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.